1. Field of the Invention
The present invention relates to a power supply apparatus for transferring an image in an image forming machine, and more particularly, to a power supply apparatus for transferring an image in an image forming machine for supplying power to a transfer roller and a transfer backup roller for transferring an image which is created on an intermediate transfer medium by a plurality of photoreceptors to a print medium.
2. Description of the Related Art
Generally, one of the typical apparatuses for forming an image on a print medium using an intermediate transfer medium is a single pass color image forming machine. The single pass color image forming machine has a plurality of color image forming units each forming an individual color image on the intermediate transfer medium in a single pass. A color image forming machine creates a predetermined image by precisely overlapping color images transferred from each color image forming unit on the intermediate transfer medium and transfers the created image onto a print medium. Conventionally, the plurality of color image forming units are consecutively arranged in the order of the intermediate transfer medium moving direction respectively developing images in cyan C, magenta M, yellow Y, and black B. The color image forming machine in the present description refers to an apparatus forming a color image on a print medium such as a color printer or a color copier.
An example of an image forming machine for printing an image using such an intermediate transfer medium is shown in FIG. 1.
Referring to FIG. 1, the image forming machine comprises a plurality of color image forming units 10C, 10M, 10Y, 10K, an intermediate transfer medium 30, a transfer device 40, and a transfer power supply apparatus 50.
The plurality of color image forming units 10C, 10M, 10Y, 10K are arranged in the order of the moving direction (shown by the arrow) of the intermediate transfer medium 30 forming images respectively in cyan C, magenta M, yellow Y, and black B. Each color image forming unit 10C, 10M, 10Y, 10K is provided with a photosensitive medium transferring an image onto the intermediate transfer medium 30, a charging roller (not shown) charging the photosensitive medium with a very high voltage, and a development roller (not shown) developing an electrostatic image created on the photosensitive medium in each color. An intermediate transfer roller 20C, 20M, 20Y, 20K is provided for each color image forming unit 10C, 10M, 10Y, 10K having the intermediate transfer medium 30 interposed therebetween for transferring the image created by each color image forming unit 10C, 10M, 10Y, 10K to the intermediate transfer medium 30.
Each color image created by the plurality of color image forming units 10C, 10M, 10Y, 10K is consecutively overlapped on the intermediate transfer medium 30, thereby creating a predetermined color image, and the intermediate transfer medium 30 conveys the color image to the transfer device 40. The intermediate transfer medium 30 is comprised of polyamide, polycarbonate, and urethane which are resilient.
The transfer device 40 comprises a transfer roller 41 and a transfer backup roller 42, for transferring the color image created on the intermediate transfer medium 30 onto a print medium 60 supplied from a feed device (not shown).
The transfer roller 41 allows the image formed on the intermediate transfer medium 30 to be transferred onto the print medium 60 by supplying a constant voltage while pressing the print medium 60 against the intermediate transfer medium 30 by a predetermined pressure. The transfer roller 41, which is a resilient body with a low degree of rigidity, presses the intermediate transfer medium 30 with a low pressure and is electrically formed to be supplied with a high voltage.
The transfer backup roller 42 is comprised of a resilient body with a low degree of rigidity or an electroconductive metal, and supports the intermediate transfer medium 30 pressed by the transfer roller 41. The transfer backup roller 42 is grounded.
The transfer power supply apparatus 50 supplies a predetermined transfer voltage to the transfer roller 41 in order to form an electrostatic field for transferring the image created on the intermediate transfer medium 30 onto the print medium 60. The transfer power supply apparatus 50 comprises a positive power unit supplying positive polarity voltage to the transfer roller 41, a negative power unit supplying negative polarity voltage to the transfer roller 41, and a power shifting member selectively connecting the transfer roller 41 with the positive power unit or the negative power unit. The transfer power supply apparatus 50 controls the power shifting member in order to reverse the polarity of the power supplied to the transfer roller 41 relative to the image during the transfer process when the image on the intermediate transfer medium 30 is being transferred onto the print medium 60 and to once again reverse the polarity during the cleaning process when no image is transferred onto the print medium 60. That is, the transfer power supply apparatus 50 supplies a voltage having the opposite polarity to the polarity of the image created on the intermediate transfer medium 30 for the image to be transferred onto the print medium 60 during the transfer process and supplies the same polarity to that of the polarity of the image created on the intermediate transfer medium 30 so that the image is not transferred to the transfer roller 41 during the cleaning process.
Therefore, visible images created on the photoreceptors of the plurality of color image forming units 10C, 10M, 10Y, 10K through charge, exposure, and development processes are transferred onto the intermediate transfer medium 30 and are precisely overlapped with each other by the electrostatic force formed between the intermediate transfer rollers 20C, 20M, 20Y, 20K and the photoreceptors. The image created on the intermediate transfer medium 30 is transferred again onto the print medium 60 by the transfer roller 41 and the transfer backup roller 42. Since the transfer roller 41 presses against the intermediate transfer medium 30 by a predetermined pressure, and the transfer power supply apparatus 50 supplies a predetermined transfer voltage to the transfer roller 41 transferring development agent to the print medium 60, the development agent such as ink or toner forming an image on the intermediate transfer medium 30 is effectively transferred to the print medium 60.
During the cleaning process when no image is transferred from the intermediate transfer medium 30 to the print medium 60, the transfer power supply apparatus 50 supplies a transfer voltage to the transfer backup roller 42, which is the same transfer voltage transferring the image created on the intermediate transfer medium 30 to the print medium 60. Accordingly, the development agent on the intermediate transfer medium 30 does not contaminate the transfer roller 41 when no image transfer is made to the print medium 60.
However, when the temperature around the image forming machine varies, resistance of the intermediate transfer medium 30 or the transfer roller 41 varies, and when the resistance of the intermediate transfer medium 30 or the transfer roller 41 varies, the output of the transfer power supply apparatus 50 varies. When the output of the transfer power supply apparatus 50 varies, the quantity of the development agent being transferred from the intermediate transfer medium 30 to the print medium 60 varies, thereby causing a problem that the quality of the image transferred onto the print medium 60 varies depending on the surrounding environment. In other words, the conventional transfer power supply apparatus of an image forming machine cannot maintain a predetermined degree of quality of the image transferred onto the print medium regardless of the change in the external environment.
In addition, since the conventional transfer power supply apparatus 50 supplies a necessary voltage by connecting the transfer roller 41 either to the negative power unit or the positive power unit using the power shifting member when switching the mode from the transfer process to the cleaning process or vice versa, the output voltage loaded in the transfer roller 41 largely varies, thereby causing poor quality of the image transferred on the print medium 60 or ineffective cleaning of the intermediate transfer medium 30.
Therefore, there is a need for a transfer power supply apparatus of an image forming apparatus which can maintain the stable quality of the print image by supplying an appropriate voltage to a transfer roller according to the change in the temperature of the surrounding environment during the transfer process, and supplying an appropriate voltage to a transfer backup roller according to the change in the temperature of the surrounding environment during the cleaning process.